This invention relates to methods and apparatus for controlling the composition of the atmosphere within a container, in particular refrigerated containers during the storage and/or transportation of produce which is perishable and which may respire, such as fruit, vegetables and flowers. (The word xe2x80x9ccontainerxe2x80x9d as used herein includes not only individual refrigerated containers such as standard ISO transportable containers but also enclosed stores, parts of warehouses, ships holds and the like).
It is known that such perishable and/or respiring produce may be carried in refrigerated containers, and refrigeration units have been developed for use with such containers, which units are reliable and function efficiently for long periods of time.
It is also known that perishable and/or respiring produce is affected by the surrounding atmosphere, and that, by modifying the composition of the atmosphere surrounding the produce, its preservation in storage or during transportation can be enhanced. Specific atmospheric components which are usually considered are hydrocarbons, carbon dioxide, oxygen and water vapour. Hydrocarbons are evolved by some types of fruit and vegetable, and these can promote rapid ripening and are therefore undesirable in storage/transportation atmospheres; the principal hydrocarbon is ethylene (C2H4), and hereinafter and in the accompanying claims the use of the word xe2x80x9cethylenexe2x80x9d should be considered to embrace all hydrocarbons, whether evolved by respiring produce or present in ambient air. Carbon dioxide is also produced by respiring products, and is known to have either an adverse or a favourable effect on the preservation of such products. Oxygen and water vapour are also known to affect the preservation of many perishable products, and generally a lower than atmospheric level of oxygen is useful for preserving perishable products.
A problem in controlling the atmosphere surrounding perishable and/or respiring produce in a refrigerated container is that the atmospheric requirements for optimum preservation vary between different kinds of fruit and vegetable. Also, different fruits and vegetables respire at different rates and evolve differing amounts of carbon dioxide, ethylene and/or water vapour. For example, whilst most fruits and vegetables require a carbon dioxide level of about 5% (for example apples and pears about 1% to 3%, cabbage about 3% to 6%), some need a significantly higher level, such as raspberries and strawberries which generally need 15% to 20% carbon dioxide. Similarly, optimum oxygen levels can vary between 2% to 3% (for examples olives, apricots and broccoli) and 5% to 10% (for example figs, lemons and peppers). High respiring products, such as apples and pears can rapidly reduce oxygen levels in a closed container below the levels required for optimum preservation. Relatively high levels of relative humidity are preferred for the preservation of most perishable products; however excessive levels of relative humidity are undesirable as they can promote rottingxe2x80x94although in practice overly high levels of relative humidity are unlikely to occur in the refrigerated atmosphere within a refrigerated container. On the other hand, unduly low levels of relative humidity are usually harmful as they promote dehydration of the produce, which can adversely affect both weight and quality. Accordingly, there is a need for a system to control the atmosphere within a refrigerated container and which is capable of adapting easily to the differing atmospheric requirements of different products, whilst being efficient, effective and relatively simple and inexpensive to manufacture and operate.
In accordance with one aspect of the invention, there is provided a method of modifying the composition of the atmosphere within a container during the refrigerated storage and/or transportation of perishable and/or respiring produce therein, comprising the following steps:
a) withdrawing and pressurising a portion of said refrigerated atmosphere;
b) adsorbing water vapour from said pressurised atmosphere by passing it concurrently through a layer of an adsorbent which preferentially adsorbs water vapour from said atmosphere;
c) adsorbing carbon dioxide from the water vapourxe2x80x94depleted atmosphere by passing it concurrently through a layer of an adsorbent which preferentially adsorbs carbon dioxide from the atmosphere;
d) adsorbing ethylene from the water vapour and carbon dioxide-depleted atmosphere by passing it concurrently through a layer of an adsorbent which preferentially adsorbs ethylene from the atmosphere;
e) returning the water vapour, carbon dioxide- and ethylene-depleted atmosphere to the container, and
f) regenerating the adsorbent layers by passing ambient air therethrough in a countercurrent direction and thereafter venting said air to ambient atmosphere, wherein water vapour is adsorbed from said ambient air by passing it through a layer of an adsorbent which preferentially adsorbs water vapour from said air before it passes through said ethylene and said carbon dioxide adsorbent layers.
The method is preferably carried out with the above steps being conducted in the order stated.
Such a method (in which the adsorption/desorption processes are preferably carried out on a pressure-swing basis, although those skilled in the art will appreciate how they could be effected on a temperature-swing basis, both pressure-swing and temperature-swing modes of operation being well documented in the art of separating air or gas mixtures into their component parts) is advantageous because it enables the minimum number of separate adsorption processes and/or the minimum size of adsorption apparatus to be used for effective atmosphere control, which (because most adsorption processes do not effect a total separation of each component from the atmosphere) minimises the loss of atmosphere components which are needed to be returned to the container. The method is therefore very efficient. In addition, because it is possible to minimise the number of adsorption processes, it is possible to provide a system which is sufficiently compact to fit within the confines of a standard container but without occupying too much of the available storage space. In particular, the initial adsorption of water vapour and carbon dioxide (in steps b) and c)) can be effected in a single layer of adsorbent material. Additionally or alternatively, all the adsorption steps can be effected in a single adsorption vessel, containing several layers of adsorbent.
Because the ambient air will nearly always contain significantly more water vapour than the refrigerated atmosphere within the container, it is necessary to dry the air for regenerating the adsorbent layers before it reaches the carbon dioxide adsorbent layer (so as to protect the adsorbent material therein) and, optimally, before it reaches the ethylene adsorbent layer; contamination of these layers with water would be likely to degrade their adsorption characteristics to an unacceptable and even irremediable extent.
As part of the process of regenerating the adsorbent layers, the adsorption vessel may be vented to atmosphere prior to the regeneration step (i.e. between steps e) and f)).
During the removal of carbon dioxide, particularly from the atmosphere withdrawn from a container holding produce with a high respiration rate, a partial vacuum is created within the container. Because in practice it is not feasible to make standard containers completely sealed to the ambient air, this vacuum draws ambient air into the container, significantly raising at least the oxygen level within the container and so adversely affecting the preservation of the produce.
Consequently, the method may further comprise pressurising ambient air and depleting it of water vapour, oxygen, carbon dioxide and ethylene by adsorption thereof, cyclically with steps a) to f), and passing the resulting nitrogen-rich atmosphere into the container.
As can be appreciated, cycling the injection of nitrogen-rich atmosphere into the container with the successive adsorption steps to scrub the container atmosphere of carbon dioxide, oxygen and ethylene, enables a single compressor to be utilised. The compressor used for the adsorption processes can also be used, cyclically, to drive the adsorption process producing the nitrogen-rich atmosphere from ambient air. Injection of the nitrogen-rich atmosphere into the container is advantageously carried out until the pressure within the container is somewhat above ambient. The resulting over pressure, whilst tending to leak to the ambient atmosphere surrounding the container, will prevent ambient air from entering the container and affecting the composition of the atmosphere therein.
To produce the nitrogen-rich atmosphere, ambient air is preferably compressed and passed through a bed containing at least a layer of alumina, a layer of activated carbon and a layer of carbon molecular sieve. Water vapour, oxygen, carbon dioxide and ethylene, as well as other contaminants in the ambient air, are adsorbed and the resulting nitrogen-rich mixture injected into the container. The nitrogen generation system can operate as a conventional two bed system, involving production, equalisation and regeneration steps as is known in the art (the nitrogen-producing unit being regenerated by venting and then purging with nitrogen-rich product gas), or as a single bed system. In the latter case, a nitrogen receiver or buffer vessel will be required for storing nitrogen-rich gas for use in regenerating the bed; the advantage of a single step system is that the number of valves in the system is minimised.
Advantageously, the nitrogen generator, whether a single or two bed system, can be arranged so as to feed not directly into the container but rather into the adsorption vessel for adsorbing carbon dioxide and/or ethylene prior to the introduction of the nitrogen into the container. With such an arrangement, the nitrogen-rich atmosphere can be stripped of any carbon dioxide and/or ethylene so as to ensure no unintentional and undesirable addition of these components to the container atmosphere.
In order to control the atmosphere within the container, the method may further comprise sensing the levels of carbon dioxide, pressure and oxygen within the container, comparing the sensed levels with predetermined levels, the predetermined levels of carbon dioxide and oxygen being determined by the nature of the produce within the container, and initiating and controlling the steps a) to f), the passing of nitrogen-rich atmosphere into the container, the injection of carbon dioxide into the container, and/or the venting of the container to ambient atmosphere until the sensed levels are substantially equal to the predetermined levels. As will be described further below, this facilitates the provision of a single and reliable control system to modify the atmosphere within the container to any of a wide range of atmosphere compositions, according to the nature of the produce carried in the container, the system responding to a primary carbon dioxide level signal, and to subsidiary pressure and oxygen level signals. In practical embodiments of the invention, it is envisaged that the predetermined level of carbon dioxide would vary between 0 and 12% (with a tolerance of about 50%, but preferably no more than about 20%) and the predetermined level of oxygen would vary between 2 and 21% (with a similar tolerance), according to the requirements of different products. The predetermined pressure level would be above ambient, in order to prevent the ingress of ambient air, but an amount determined by such variable factors as the container leak rate, the cycle rate of the nitrogen injection, and so on. In practice, a predetermined pressure level of approximately 2 inches water gauge (500 Pa) would usually be sufficient.
The method may further comprise sensing the level of relative humidity within the container, comparing the sensed relative humidity with a relative humidity level determined according to the nature of the produce within the container and, when the sensed level is less than the predetermined level, initiating and controlling the injection of atomised water into the container until the sensed level is substantially equal to the predetermined level. As previously noted, overly and undesirably high levels of humidity are unlikely to be encountered in practice and it is therefore only necessary to protect against low relative humidity.
In accordance with another aspect of the invention, the invention provides apparatus for modifying the composition of the atmosphere within a refrigerated container during the storage and/or transportation of respiring produce therein, the apparatus comprising means for withdrawing and pressurising a portion of the atmosphere and for introducing the pressurised atmosphere into at least one adsorption vessel for passage therethrough in a concurrent direction, the or each adsorption vessel containing several layers of adsorbent material, successive layers in the concurrent direction being adapted for the preferential adsorption of water vapour, carbon dioxide and ethylene, means being provided for returning the water vapour, carbon dioxide and ethylene-depleted atmosphere to the container, and means being provided for passing pressurised ambient air through the layers of adsorbent material in the adsorption vessel in a countercurrent direction, a further layer of adsorbent material which preferentially adsorbs waters vapour being provided directly adjacent the layer of adsorption material which preferentially adsorbs ethylene.
A single layer of an adsorbent material which preferentially adsorbs both water vapour and carbon dioxide may be provided, and located directly adjacent the layer of adsorbent material which preferentially adsorbs ethylene.
Alternatively, the further layer of adsorbent material is interposed between and directly adjacent the layers of adsorption material which preferentially adsorb carbon dioxide and ethylene.
The separate layers of adsorbent material in the or each adsorption vessel are preferably held packed contiguously by compression means. In the case where the adsorption vessel is cylindrical, the compression means comprises a sprung circular plate which presses the adsorbent materials tightly together within the cylinder. The advantage of such an arrangement is that it enables the adsorption vessel to be disposed in any orientation within the containerxe2x80x94vertically, horizontally or upside-downxe2x80x94without upsetting the layers of adsorbent material, so that the adsorption processes are unaffected.
Means may be provided for injecting carbon dioxide into the container, to modify the atmosphere therein when the carbon dioxide level is too low.